The present invention may be more easily understood in the context of low light imaging arrays such as those used in digital photography to record an image. For the purposes of this discussion, an image will be defined as a two-dimensional array of digital values that represent the amount of light received during an exposure period at each pixel on a two-dimensional surface onto which the image is projected. For the purposes of this discussion, it will be assumed that each pixel is a small rectangular area on that surface. In digital photography, the image is projected onto an imaging array in which each pixel includes a photodetector that measures the amount of light that falls on some portion of the pixel area.
The quality of the image is set by the signal to noise ratio at each pixel. The signal is proportional to the number of photons that were converted to electrons at that pixel. As the light levels in the image decrease, i.e., low light applications, the number of available photons at each pixel eventually becomes the limiting factor. If the number is too low, each pixel will have “statistical noise” that is approximately equal to the square root of the number of photons that were converted to electrons. Hence, the pixel parameters must be chosen to assure that a sufficient number of photons are converted at each pixel location.
The ratio of the portion of each pixel in which the light is converted to electrons to the total pixel area is referred to as the “fill factor”. The number of photons converted at each pixel depends on the exposure time, the pixel area, the fill factor, and the probability that a photon striking the active part of the pixel actually generates an electron that is captured by the photodetector. There is an upper limit on the exposure time that is set by the scene being imaged. The exposure time must be sufficiently small to “freeze” any motion in the scene. The area of a pixel is likewise constrained by the degree of resolution required in the image, since large pixel areas lead to grainy images. The probability that a photon will be converted to an electron is a property of the material from which the array is constructed, and hence, is not easily changed. Accordingly, imaging arrays having large fill factors are preferred for low light applications.
In one class of imaging array, the detector utilizes an area of silicon to collect electrons that are generated by light that strikes the silicon. During the exposure period, the electrons accumulate in the pixel area. The charge collected in each pixel area is measured at the end of the exposure period by moving the charge to an amplifier and an analog-to-digital converter that provides a digital value for each pixel. The pixels are arranged as a plurality of columns of pixels. Each pixel in a column is part of an analog shift register. The image is readout by shifting the charge collected at each pixel through the shift register until it reaches the end of the column. The charge is then either input to an amplifier or moved to another shift register that finally deposits the charge at the amplifier. Imaging arrays of this type are often referred to as charge-coupled devices (CCDs). CCDs are characterized by large fill factors, since most of the area of each pixel is devoted to generating and storing electrons from the incident light, and hence, such devices have the potential for providing imaging arrays that can operate under low light conditions.
However, other sources of noise degrade the performance of these devices. Since the individual pixels have no amplification circuitry at the pixel, the small charge signals must be moved relatively long distances to the readout amplifiers that are shared by a large number of pixels. The amplifier may be viewed as having a capacitor that is charged by the charge collected by the pixel that is currently being processed. The capacitor must be reset to a known voltage level prior to receiving the charge in question. The charge is then transferred to the amplifier and the voltage across the capacitor is amplified and digitized. Any error in the voltage to which the capacitor is reset between charge measurements contributes to the noise in the measured values. In low light applications, the available charge is already small, and hence, this noise source can limit the performance of the imagining array.